Various processes and systems for the manufacture of acrylonitrile and methacrylonitrile are known; see for example, U.S. Pat. No. 6,107,509. Typically, recovery and purification of acrylonitrile/methacrylonitrile produced by the direct reaction of a hydrocarbon selected from the group consisting of propane, propylene or isobutylene, ammonia and oxygen in the presence of a catalyst has been accomplished by transporting the reactor effluent containing acrylonitrile/methacrylonitrile to a first column (quench) where the reactor effluent is cooled with a first aqueous stream, transporting the cooled effluent containing acrylonitrile/methacrylonitrile into a second column (absorber) where the cooled effluent is contacted with a second aqueous stream to absorb the acrylonitrile/methacrylonitrile into the second aqueous stream, transporting the second aqueous stream containing the acrylonitrile/methacrylonitrile from the second column to a first distillation column (recovery column) for separation of the crude acrylonitrile/methacrylonitrile from the second aqueous stream, and transporting the separated crude acrylonitrile/methacrylonitrile to a second distillation column (heads column) to remove at least some impurities from the crude acrylonitrile/methacrylonitrile, and transporting the partially purified acrylonitrile/methacrylonitrile to a third distillation column (product column) to obtain product acrylonitrile/methacrylonitrile. U.S. Pat. Nos. 4,234,510; 3,936,360; 3,885,928; 3,352,764; 3,198,750 and 3,044,966 are illustrative of typical recovery and purification processes for acrylonitrile and methacrylonitrile.
Effluent from the quench column may be cooled further before transporting to other downstream equipment. In one aspect, effluent from a quench is cooled in an indirect contact cooler, called a quench aftercooler (QAC) prior to going to an absorber column. The QAC is typically a vertical, shell-and-tube exchanger with the process effluent flow through the tube side and cooling medium on the shell side. The effluent vapor cools as it travels through the tubes and some organics (primarily acrylonitrile) and water condense, this is called process condensate. Uncondensed vapor exits the bottom of the QAC through a nozzle in the side of the exchanger below the tube sheet. The process condensate exits the bottom of the QAC under level control and is pumped to downstream equipment (absorber or recovery column).
The process may experience a problem of blockage of the QAC tubes that requires periodic shutdown of the plant for mechanical cleaning of the QAC. The blockage is due to gradual buildup of polymer on the inside of the tubes. The polymer is primarily poly-AN. The reason for the polymerization is that some acrylonitrile (AN) condenses on the tubes and this AN monomer is not inhibited, which readily allows polymerization to occur. Furthermore, the quench effluent contains some ammonia which is not removed in the quench, and ammonia reacts with AN in liquid phase condensate to form polymer. The AN polymer is sticky and some can adhere to the inside tube walls and gradually build up leading to blockage of the tubes.